<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        var first = true;
        function a(){
            b();
            console.log('a');
        }

        function b(){
            if(!first){
                a();
            }
            console.log('b');
        }
        a()
    </script>
</body>
</html>